1. Field of the Invention
The present invention relates to a method and to an apparatus for making structural reinforcement preforms which are subsequently used for molding processes such liquid composite molding (LCM) as resin transfer molding or other processes requiring preforms for structural composites. It is further particularly concerned with the structural modification of preforms by attachment of specific reinforcement members, by being joined to other preforms, or where the preforms are modified in shape, all by a process of energetic stitching.
2. Description of the Prior Art
In the manufacture of preforms as done by the prior art, the processes have been time-consuming and relatively expensive. In making preforms according to one process known as a directed fiber process, it has heretofore been the practice to spray chopped fibers with a binder resin onto a form that has air pulled therethrough to locate and hold the fibers in place. The form with the fibers and the binder resin thereon is then moved into a hot air plenum chamber, is dried and/or cured with heat to set the binder resin. In addition, a great deal of processing space is required for drying, curing and cooling the preforms.
In making preforms by another process known as the thermoforming process, it has heretofore been the practice to use a material such as a continuous strand fiber mat that has been previously coated by the fiber manufacturer with a thermoplastic binder. The thermoformable material is supplied typically in a roll form whereby it is unrolled into flat sheets of varied layer thicknesses and clamped into a holding frame at the edges. The frame network is then positioned in an oven chamber containing radiant heaters which slowly heat the reinforcement mat and the thermoplastic binder from both sides. Upon heating, the thermoplastic binder softens and, while soft, the frame network is quickly transferred into a cold mold. The mold closes via a press forcing the reinforcement mat into the desired shape of a preform. Upon cooling, the thermoplastic binder stiffens and thus holds the thermoformable mat in its new shape. There are other machinery arrangements to accomplish thermoforming but the steps are the same.
As pointed out in an earlier copending patent application, Ser. No. 07,446,859, filed Dec. 6, 1989, these processes are slow, require a great deal of space and a large amount of energy. As also pointed out, in conventional LCM process applications for structural components, fiber layer thickness across the entire preform is increased to meet the strength requirements of one area, which results in unnecessary use of material in other areas and increases thickness and weight. Furthermore, neither the directed fiber process nor the thermoformable mat process allows a designer to add ribs or closed sections to maximize design properties.
In the aforementioned application, therefore, there has been proposed a new system which eliminates the necessity for large rooms and constantly operating ovens, cooled presses and the like and permits design flexibility with respect to the provision of reinforcement ribs, closed sections, placement of specific reinforcements and/or attachment members and complex shapes while at the same time saving on energy and materials.
The new process utilizes specifically-developed binders along with directed energy systems for rigidizing the composite forms and attaching structural components to the preforms and is entirely compatible with RTM and SRIM resin systems, i.e. polyesters, vinyl esters, urethanes, epoxies and phenolics. The process is designed to be capable of full automation and to enable specific distribution and placement of numerous type of reinforcements, where necessary, for the required structural properties of a preform. There is a complete freedom of design inherent in the process and allows for the most desirable reinforcement type and/or structures including closed structural shapes and varied wall sections to meet design criteria.
In the new process, mats of reinforcement material are cut into a desired shape as a two-dimensional planar development of a desired preform. The cut mats are then coated with a binder which is responsive to electromagnetic energy, either microwave radiation or ultraviolet radiation, and the cut mats are placed in a three-dimensional mold and pressed to replicate the desired shape of the preform.
While in the mold, the coated and shaped mats are subjected to the appropriate electromagnetic radiation, either microwave or ultraviolet radiation, to cure the binder resin and provide rigidity in a matter of seconds, rather than minutes as with the heat-curable processes. At this point, the preform is an intermediate product ready for use in a further molding operation (LCM) or it may be used as a carrier preform for the attachment of additional structural reinforcement members and the like before being used in a further molding operation (LCM).
As a carrier preform, this preform can be modified by an energetic stitching process. Such modification may involve the attachment of a specific reinforcement member, the attachment of another preform or a modification of the preform shape. For example, where the preform has a very complex shape, several subassembly preforms can be separately made and joined to each other by energetic stitching to have the final form of the complex preform. As an example of one manner of using the energetic stitching process where a reinforcement member or rib is to be added, the rigid three-dimensional preform is moved to a station where a designated area or areas of the preform or of a subassembly have been masked and therefore uncured or are provided with a further coating of an electromagnetic energy curable binder resin, the subassembly (e.g. reinforcement member or the like) is moved into intimate contact with the preform at the coated area or areas and the appropriate electromagnetic radiation is applied to energetically stitch (cure the binder) the subassembly to the carrier preform. This can also take place in situ when the final attachment has been made by such energetic stitching, the preform is ready for use as a structural reinforcement preform as a part of a further molding process for making a structural composite.
In structural composites, it is often necessary to have closed sections to maximize the properties required of the molded part at minimum weight. This requires the use of lightweight core materials to be included in the preform since a hollow preform will fill up with resin or collapse during the molding process. Therefore, in the modification of the preform, a core may be inserted in any such pocket or chamber to prevent or at least minimize such an event. The core may be held in place by a subassembly or sequential assembly of the preform and energetically stitched to surround the core. This eliminates the use of staples, mechanical stitching or tapes to fasten to the core. The core material can be any lightweight material used in the conventional art such as balsa wood, urethane foam, phenolic foam, PE foam or can be of a pressurized bladder that is removable and left in or blow molded hollow core pressurized sufficiently to withstand the liquid resin pressure during the molding phase.